Ag-Ni偏晶合金凝固过程研究

Study on the Solidification of Ag-Ni Monotectic Alloy

对Ag-Ni偏晶合金开展了快速/亚快速凝固实验,获得了富Ni相粒子均匀弥散分布于Ag基体的合金样品,Ag-Ni合金显微硬度随着合金Ni含量增加和试样凝固过程冷却速率升高而增大,当Ag-4.0%Ni合金液-液相变开始阶段熔体冷却速率达1800 K/s时,其显微硬度接近粉末冶金生产的Ag-10.0%Ni片状电触头的硬度。建立了描述Ag-Ni合金凝固组织演变的动力学模型,模拟计算了Ag-Ni合金凝固组织形成过程,分析讨论了合金成分和试样直径(冷却速率)对Ag-Ni合金凝固组织形成过程的影响。结果表明:富Ni相液滴/粒子形核阶段熔体的冷却速率对合金凝固组织弥散度具有决定性影响;合金的Ni含量越高、试样冷却速率越低,凝固组织中富Ni相粒子平均尺寸越大;Ag-Ni合金熔体冷却凝固时,富Ni相液滴/粒子的尺寸主要受形核和长大控制,Ostwald粗化作用很弱。

The Ag-Ni alloy has high electrical conductivity,good thermal conductivity,high specific heat capacity,and excellent electrical wear resistance if the Ni-rich phase is dispersedly distributed in the Ag-based matrix.It has been widely used in the medium load contactors,magnetic starters,relays,etc.However,Ag-Ni alloy is a typical monotectic system.Generally,the liquid-liquid phase transformation leads to the formation of a solidification microstructure with serious phase segregation.So far,there have been few studies on the solidification process of Ag-Ni alloys and powder-metallurgical techniques are commonly used to prepare Ag-Ni alloys in industry.In this work,casting experiments and microhardness test were carried out with the Ag-Ni monotectic alloy.The samples with composite microstructure,in which the Ni-rich particles dispersed homogeneously in Ag matrix,were obtained.The microhardness of Ag-Ni alloy increases with the increase of nickel content and the cooling rate of the sample during solidification.When the cooling rate during the liquid-liquid phase transition of the Ag-4.0%Ni alloy reaches1800 K/s,the microhardness of the Ag-4.0%Ni alloy is close to that of the Ag-10.0%Ni sheet electrical contacts produced by powder metallurgy.A model describing the microstructure evolution during cooling Ag-Ni monotectic alloy melt has been proposed.The process of microstructure formation has been simulated and discussed in details.The results indicate that the cooling rate during the nucleation of the Nirich droplets/particles has a dominant influence on the solidification microstructure.The average radius of the Ni-rich particles increases with the increase of nickel content,while it decreases with the increase of the cooling rate during solidification.The average radius of the Ni-rich particles shows an inverse square root dependence on the cooling rate during the nucleation of the Ni-rich droplets/particles.The Ostwald coarsening of the Ni-rich droplets/particles is very weak during cooling Ag-Ni monotectic alloy melt.Rapid/sub-rapid solidification has a good application prospect in the preparation of the high-performance Ag-Ni contact materials.